The present invention relates to a scanning electron microscope, and in particular, a scanning electron microscope which uses an imaging gas.
Presently, a number of types of scanning electron microscope are provided, such as the ESEM (Environmental Scanning Electron Microscope), the VPSEM (Variable Pressure Scanning Electron Microscope) and the LPSEM (Low Pressure Scanning Electron Microscope) which operate using an imaging gas to amplify secondary electrons produced by the target sample. Such systems operate by irradiating the target sample with an electron beam which in turn causes the emission of electrons from the sample, which can include the emission of secondary electrons. The secondary electrons are then accelerated towards an anode, which can be used as a detector.
The target sample is placed in a low pressure sample chamber which includes a gas such as water vapour. As the secondary electrons are accelerated towards the detector, the electrons collide with water vapour causing the production of ions and additional electrons. The ions fall back to the surface of the sample and act to compensate for any negative charge which has been induced by the irradiating electron beam. Meanwhile, the additional electrons are accelerated towards the anode, which causes a cascade effect such that for each secondary electron emitted by the sample, a number of electrons are incident on the anode.
By scanning the irradiating electron beam across the sample surface and measuring an appropriate signal, such as the number of electrons incident on the anode at each position, an image of the sample surface can be generated.
However, not all the ions produced operate to neutralise the build up of charge on the sample. Accordingly, a number of ions are typically present in the sample chamber and this can lead to a reduction in the quality of the image generated, for example due to recombination of the ions with electrons, and/or the presence of a space charge within the sample chamber which can lead to a reduction in the acceleration of the electrons towards the anode.
A solution to this has been proposed in U.S. Pat. No. 5,396,067 which describes the addition of a ion collector plate. The ion collector plate is positioned in the sample chamber between a pressure limiting aperture and the sample. The plate is charged to a positive potential below that of the pressure limiting aperture, to ensure amplification between the plate and the sample. As a result, any ions generated within the sample chamber fall back on to and are collected by the ion collector plate.
However, with the ion collector positively charged, not all the ions will be collected and accordingly, the sample will still be subject to a build-up of positive charge. Furthermore, the collector plate includes only a single aperture through which the irradiating beam must pass. This therefore can restrict the field of view to the sample. Furthermore, the presence of the collector plate can obstruct electrons emitted from the sample surface, as well as preventing additional detectors positioned in the sample chamber from monitoring the sample.
An alternative system has been proposed in U.S. Pat. No. 5,466,936 in which an ion detector is provided in the roof of the sample chamber adjacent the secondary electron detector. In this example, ions generated in the region of the secondary electron detector are collected by the ion detector and are then used in the generation of the image signal. This utilises the fact that the ions are generated by the secondary electrons, and accordingly the number of ions incident on the ion detector is also representative of the number of secondary electrons emitted from the sample.
Accordingly, by adding the signal representative of the number of ions collected by the ion collector to the signal obtained from the electron detector can lead to the production of a combined signal which can be used in the generation of an image. However, this combined signal merely increases the magnitude of the signal used by the image processing system. Accordingly, this combined signal does not result in an enhanced image which includes additional information, it is merely stated that the signal-to-noise ratio is improved when compared to an image obtained using secondary electron detection only.
In accordance with a first aspect of the present invention, we provide a scanning electron microscope for imaging a sample, the microscope comprising:
a. a sample chamber containing a gas in which the sample is positioned in use,
b. a bias member which is maintained at a predetermined electrical potential so as to accelerate electrons emitted from the sample; and,
c. an ion collector comprising one or more electrically conductive elongate members extending into a region between the sample and the bias member, the or each elongate member being maintained at a potential below the predetermined electrical potential to thereby collect the ions.
Accordingly, the present invention provides apparatus for reducing the effect of ion build up in the sample chamber of a scanning type electron microscope. This is achieved using a ion collector which is held at a predetermined potential and therefore operates to decouple the field generated by the bias member from the sample. This ensures that electrons emitted from the sample are always accelerated by a constant field between the ion collector and the bias member. Furthermore, excess ions generated within the sample chamber are discharged by the ion collector thereby reducing the effects of electron recombination and charge cloud formation in the sample chamber. In addition to this, because the ion collector is formed from one or more elongate members it therefore has a relatively small cross-sectional area compared to the sample. As a result, the ion collector does not obstruct the irradiating electron beam or prevent secondary electrons being accelerated away from the sample, whilst still allowing the absorption of ions. Furthermore, by using an ion collector with a relatively small cross-sectional area, this ensures that the view of the sample is not blocked from elsewhere within the sample chamber. This allows additional detector equipment, such as X-ray detectors, backscatter electron detectors, and cathode luminescence detectors to be used within the sample chamber.
In one example, an image of the sample is generated by a detection system which uses a sensor coupled to the bias member for determining the number of electrons incident thereon; and, a processing system responsive to the sensor to generate an image of the sample. In this case, because the ion collector decouples the field from the sample, this ensures that the secondary electrons emitted from the sample surface are accelerated towards the bias member thereby causing a cascade effect which ensures a large number of electrons are incident on the bias member. Furthermore, with space charge and recombination effectively diminished, the image generated by the signal obtained from the bias member is greatly enhanced over the image obtained without the ion collector present. This enhancement allows features to be viewed which are not normally visible using this form of detection.
Alternatively, the detection system may also comprise:
i. the ion collector;
ii. a sensor coupled to the ion collector for determining the number of ions collected thereon; and,
iii. a processing system responsive to the sensor to generate an image of the sample.
In general, the image generated by determining the number of ions collected by the ion collector results in an image of similar quality to the enhanced electron image (i.e. an image of far greater quality than is normally detected when only a secondary electron detector is present). Both the enhanced electron image and the ion image allow the features to be viewed which are not normally discernable on images produced by a secondary electron detector.
In accordance with a second aspect of the present invention, we provide a scanning electron microscope for imaging a sample, the microscope comprising:
a. a sample chamber containing a gas in which the sample is positioned in use,
b. a bias member which is maintained at a predetermined electrical potential so as to accelerate electrons emitted from the sample; and,
c. a detection system for generating the image of the sample, the detection system comprising:
i. an ion collector positioned between the sample and the bias member, the ion collector being maintained at a potential below the predetermined electrical potential to thereby collect the ions from the region between the sample and the bias member;
ii. a sensor coupled to the ion collector for determining the number of ions collected thereon; and,
iii. a processing system responsive to the sensor to generate an image of the sample.
The present invention can therefore also be advantageously used by coupling a detection system to the ion collector allowing a signal representative of the number of ions collected to be generated. This can then be used to generate an image of similar quality to the enhanced secondary electron image described above. Accordingly, this allows the secondary electron detectors to be removed from the apparatus, although of course an anode must be present.
It will be realized that the detection system of the second aspect of the present invention can use the ion collector used by the scanning electron microscope of the first aspect of the present invention. Thus, the ion collector would typically comprise one or more electrically conductive elongate members extending into a region between the sample and the bias member, the or each elongate member being maintained at a potential below the predetermined electrical potential to thereby collect the ions.
The greatest effect of the field generated by the bias member being coupled to the sample occurs in the region of where the irradiating beam impinges on the sample surface. Accordingly, it is preferable that at least one elongate member is positioned typically up to within 5 mm of the irradiating beam. However, alternative positions may be used if the elongate members are suitably biassed.
The elongate members may also be shaped so as to be positioned a constant distance from the sample, thereby ensuring the field is decoupled from the entire sample. The elongate members may therefore include annular rings, rectilinear members, or the like.
Typically the or each elongate member has a cross-sectional area of below 1.0 mm2. Thicker members can be used, although these may potentially mask the imaging of the sample, and are therefore preferably avoided.
The or each elongate member is preferably maintained at a potential of 0 volts or below. This helps ensure that the positive ions generated in the region are not repelled from the wires. However, positive potentials may be used, although in this case, less ions will be collected than if negative potentials are used. Accordingly, the wires are preferably negatively biassed to therefore attract positively charged ions to the elongate members.
In any event, as long as the wires are maintained at a predetermined potential, this will act to decouple the field generated in the sample chamber from the sample, irrespective of whether a sample is conductive but electrically floating, conductive and earthed, or simply insulated.
The one or more elongate members usually comprise one or more wires. Accordingly, the ion collector typically has a number of laterally spaced wires, although a single wire is also effective. Alternatively, a number of laterally spaced wires may be arranged orthogonally with other laterally spaced wires so as to form a grid arrangement.
Wires are a convenient, readily available item that are ideal for use as the ion collector. This is because the wires have a low cross-sectional area and therefore do not obstruct the irradiating electron beam, or the generated secondary electrons emitted from the sample surface. As an alternative to wires however, the system may use any form of conductive member, such as a layer of conducting material overlaid on a non-conducting substrate. In this case, the only requirement is that the substrate and the overlaid conducting material are elongate so as to have a suitably small cross-sectional area so as to allow the present invention to function correctly.
Typically the or each wire is mounted to an electrically insulating substrate. This allows the wires to be supported relative to the sample and sample chamber, whilst allowing the potential of the wires to be controlled. However, a conducting substrate may be used if positioned sufficiently far away from the irradiating beam and sample, so as not to effect microscope operation.
In this case, the wire(s) are typically arranged to extend across the aperture such that the substrate may be positioned outside the region between the sample and the bias member. In this case, as the substrate is not the active portion of the device, the aperture can be made suitably large, so that it does not mask the sample or prevent electron emission from the sample. This ensures that the substrate does not effect the imaging either by preventing the scanning beam from irradiating the sample or by preventing the acceleration of secondary electrons away from the sample surface.
Typically the ion collector is positioned between approximately 1-3 mm from the sample surface, although alternative positions may be used depending on the circumstances. Furthermore, the ion collector is typically arranged such that at least one of the electrically conductive members is positioned within 1 to 3 mm of the irradiating electron beam.
However, in alternative sample chamber arrangements, the ion collector may be positioned in alternative locations so as to ensure the satisfactory acceleration of the secondary electrons away from the sample surface.
The microscope typically further comprises:
a. a vacuum chamber;
b. a pressure limiting aperture for coupling the sample chamber to the vacuum chamber; and
c. an electron source positioned in the vacuum chamber for generating a beam of electrons, the electron source being adapted to scan the sample in use.
As set out above, the present invention utilises the realisation that sufficient ions can be removed to allow vast improvement in signal quality using only a small ion collector which is preferably either negatively biassed or grounded.